Some people have high levels of B-12 already, or high (enough) levels of folate to not need it. Too much of those things will also cause issues - so it's always best to be tested.
Kat, here is an article from a site that I keep hearing over and over again that it's the best on the web for these issues:
mthfr.net/mthfr-c677t-mutation-basic-protocol/2012/02/24/